Lightweight, highly efficient flat panel displays have been extensively studied and developed, for example, for picture display of computers and television sets.
Since Braun tubes (CRT) are high in luminance and exhibit good color reproducibility, they are most widely employed for display at present. Nevertheless, the problems are involved in that the tubes are bulky, heavy and high in power consumption.
For lightweight flat panel displays of high efficiency, there have been put on the market liquid crystal displays of such as the active matrix drive type. However, liquid crystal displays have the problems that their angle of field is narrow, they do not rely on spontaneous light and thus, need great power consumption for back light when placed in a dark environment, and they do not have a satisfactory response to high-speed video signals of high fineness which have been expected as being in use in future. Especially, a difficulty is involved in making a liquid crystal display with a large picture size, with the attendant problem of its high fabrication costs.
As a substitute therefor, a display of the type using a light-emitting diode may be possible, but such a display is also high in fabrication costs, Coupled with another problem that it is difficult to form a matrix structure of light-emitting diodes on one substrate. Thus, when considered as a candidate for a low-cost display used in place of Braun tubes, this type of display has a great problem to be solved before putting into practical use.
As a flat panel display which has the possibility of solving these problems, attention has been recently paid to organic electroluminescent devices (organic EL devices) using organic luminescent materials. More particularly, when using organic compounds as a luminescent material, it has been expected to realize a flat panel display, which makes use of spontaneous light, has a high response speed and has no dependence on an angle of field.
The organic electroluminescent device is so arranged that an organic thin film, which contains a luminescent material capable of emitting light through charge of an electric current, is formed between an optically transparent anode and a metallic cathode. In the research report published in Applied Physics Letters, Vol. 51, No. 12, pp. 913-915 (1987), C. W. Tang and S. A. VanSlyke set forth a device structure (an organic EL device having a single hetero structure), which has a double-layered structure including, as organic thin films, a thin film composed of a hole transport material and a thin film composed of an electron transport material and wherein luminescence occurs by re-combination of holes and electrons injected from the respective electrodes into the organic films.
In this device structure, either of the hole transport material or the electron transport material serves also as a luminescent material. Luminescence takes place in a wavelength band corresponding to the energy gap between the ground state and the energized state of the luminescent material. When using such a double-layered structure, a drive voltage can be remarkably reduced, along with an improved luminescent efficiency.
Thereafter, there has been developed a three-layered structure (organic EL device having a double hetero structure) of a hole transport material, a luminescent material and an electron transport material as set out in the research report of C. Adachi, S. Tokita, T. Tsutsui S. Saito and so forth, published in Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271 (1988). Moreover, a device structure comprising a luminescent material present in an electron transport material has been developed as set out in the research report of C. W. Tang, S. A. VanSlyke, C. H. Chen and so forth published in Journal of Applied Physics, Vol. 65, No. 9, pp. 3610-3616 (1989). Through these researches, evidence has been given to the possibility of luminescence of high luminance at low voltage, thus leading to recent, very extensive studies and developments.
Organic compounds used as a luminescent material are considered to be advantageous in that because of their diversity in kind, a luminescent color can be arbitrarily changed theoretically by changing their molecular structure. Accordingly, it may be easier on comparison with thin film EL devices using inorganic materials to provide, via proper molecular design, three colors of R (red), G (green) and B (blue) having good color purities necessary for full color displays.
However, organic electroluminescent devices still have problems to be solved. More particularly, a difficulty is involved in the development of a stable red luminescent device with high luminance. In an instance of red luminescence attained by doping DCM[4-dicyanomethylene-6-(p-dimethylaminostyryl)-2-methyl-4H-pyran] in tris(8-quinolinol)aluminum (hereinafter abbreviated as Alq.sub.3) for use as a currently reported electron transport material, this material is not satisfactory as a display material with respect to both maximum luminance and reliability.
BSB-BCN, which was reported by T. Tsutsui and D. U. Kim in the meeting of Inorganic and Organic Electroluminescence (at Berlin, 1996), is able to realize a luminance as high as 1000 cd/m.sup.2 or over, but is not always perfect with respect to the chromaticity for use as a red color for full color display.
Therefore, there is a need for a red luminescent device which is high in luminance, stable and high in color purity.
In Japanese Patent Laid-open No. Hei 7-188649 (Japanese Patent Application No. Hei 6-148798), it has been proposed to use a specific type of distyryl compound as an organic electroluminescent material. However, the intended luminescent color is blue, not red.
As a result, there is a need for an organic electroluminescent device, which ensures high luminance and stable red luminescence.